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Oct. 28, 2011 (Orlando, Fla.) -- Vision problems among preschoolers may be more common than previously thought.

That's the conclusion of researchers who conducted two large studies funded by the National Institutes of Health.

Previously, it was estimated that one in 20 preschoolers had vision problems. But the studies show that as many as one out of every four preschool-aged children may have a vision problem that needs attention.

The studies also show that even mild problems left untreated in childhood may lead to permanent vision loss.

The studies were published in the October issue of Ophthalmology and presented at the annual meeting of the American Academy of Ophthalmology (AAO).

In addition to the higher than expected incidence of vision problems, the studies also identify several major risk factors for vision problems in preschoolers. Included in those risks were exposure to smoke and poor access to health insurance.

Eye Exams for Preschoolers

The researchers gave eye exams to 9,970 children aged 6 months to 6 years. About 4% of the preschoolers had nearsightedness (myopia). Another 21% had farsightedness, also known as hyperopia.

Ten percent of the children had a flaw in the curvature of the eye's cornea, which caused astigmatism. Astigmatism is a problem with focusing that leads to blurred vision.

Together, these problems are referred to as refractive errors. Previous research has shown that children with moderate to severe refractive errors are at an increased risk of more serious problems. The new studies show for the first time that even mild refractive errors raise risk. This is important because refractive errors are correctible with eyeglasses.

Researchers also found more serious vision problems among preschoolers. Two percent of the preschoolers had cross eyes (strabismus). And about 5% had lazy eye (amblyopia). Both conditions can lead to permanent vision loss if not treated during early childhood.

"If not corrected right early in life," researcher Rohit Varma, MD, tells WebMD, "these kids may have vision loss for life." Varma is professor of ophthalmology and preventive medicine at the University of Southern California's Keck School of Medicine in Los Angeles.

Varma says the study showed that young children can outgrow nearsightedness and astigmatism, but not farsightedness.

AAO spokeswoman Anne Sumers, MD, tells WebMD that the link between mild refractive error and serious vision problems surprised her. Sumers is an ophthalmologist in private practice in Ridgewood, N.J. "Early screening is so very important to detect these problems."

Varma agrees. But, he says, "the eye tests given in schools are so minimal that they don't pick up many eye problems."

Varma says the diverse makeup of the children studied means the findings can be applied to U.S. preschoolers nationwide.

One limitation of the study, Varma says, is that the youngsters were only assessed at one point in time. "We need further study to see if kids with refractive errors actually go on to get vision loss and what treatments are best."